The present invention is related to the field of optical position encoders, and more particularly to optical position encoders employing techniques for suppressing undesirable harmonic components appearing in a periodic optical interference pattern within the encoder.
One general type of optical position encoder employs a source of coherent light, a diffraction grating that is movable with respect to the source, and a detector that is used to sample a pattern of interference fringes created by light from the source that is diffracted by the diffraction grating. As the grating moves with the object whose position is being measured, the pattern of interference fringes has an apparent motion by a proportional amount. The detector samples the pattern at a sufficient number of locations to create an estimate of the spatial phase of the interference fringes, which is readily convertible into an estimate of the position of the object.
In one common configuration, the optical encoder employs a so-called “four-bin” sampling and processing approach. It is assumed that the component of the fringe pattern in the direction of motion is substantially sinusoidal, which is accurate at least to a first approximation. The detector includes one or more sets of four discrete elements, and the elements of each set are arranged at 90 degree offsets from each other. The outputs of two of the elements that are separated by 180 degrees are combined to derive a value that is denoted the “sine” of the phase angle of the fringe pattern. Likewise, the outputs of the other two elements are combined to derive a value that is denoted the “cosine” of the phase angle. The position estimate is then derived as a value proportional to the arctangent of the ratio of the sine and cosine values. There are numerous variants that can be employed, including those employing other sampling arrangements such as three-bin or six-bin sampling.
In diffractive optical encoders, it is preferred that the fringe pattern be as sinusoidal as possible in order to avoid errors in the position estimate that necessarily occur when this assumption is not valid. There may be many sources of noise or other signal components that distort the fringe pattern from an ideal sinusoidal characteristic. One problem in particular is that of harmonic distortion, i.e., the presence of periodic components whose frequencies are an integer multiple of the fundamental frequency of the fringe pattern. Characteristics of the grating and/or other optical components in the encoder may introduce undesired harmonic distortion that can result in errors in position estimates. For example, encoders employing the so-called Talbot effect may have many diffractive orders that interfere at the detector to create complex fringe patterns with multiple harmonic components.
In many cases, any harmonic distortion appearing in the interference fringe pattern is filtered within a signal processor that receives the detector outputs. While this approach may be suitable in some applications, it may be difficult or impossible in others. Among other difficulties, such filtering may require an undesirably high amount of processing resources, which can lead to increased cost and other drawbacks.
It is known to use specially-designed detectors that tend to have less sensitivity to certain harmonic components of the interference fringe pattern. US Patent Application Publication 2003/0047673A1 of Thorburn et al., for example, shows a harmonic-suppressing optical detector in which the shapes, sizes and locations of individual optical detector elements are chosen such that the levels of certain harmonics, such as the third order harmonic, are reduced. In particular, the Thorburn et al. published application teaches detectors for suppressing third-order harmonics in which the widths of rectangular-shaped detector elements are equal to the spatial period T/3 of the third-order harmonic. As a result, the detector elements are insensitive to this particular component of the interference pattern, and thus the magnitude of this component in the detector output signal is very low in comparison to the magnitude of the fundamental component. The detector elements are arranged such that the four-bin processing can be performed.
U.S. Pat. No. 6,018,881 shows a magnetic position measurement system employing an oblique arrangement of magneto-resistive (MR) detector elements by which a third harmonic is filtered. The filtering reduces distortion caused by the highly non-linear response of the MR elements, which is due primarily to operation in so-called “saturation regions”.